All viruses must bind to and invade their target cells to replicate. For enveloped viruses, including RNA viruses having Class I membrane fusion proteins, the process involves (a) the binding of the virion to the target cell, (b) fusion of the envelope of the virus with the plasma membrane or an internal cellular membrane, (c) destabilization of the viral envelope and cellular membrane at the fused area to create a fusion pore, (d) transfer of the viral RNA through the pore, and (e) modification of cellular function by the viral RNA.
Steps (b) and (c) above, which involve the fusion of the viral membrane and the cell envelope, are mediated by the interaction of a viral transmembrane glycoprotein (fusion protein) with surface proteins and membranes of the target cell. These interactions cause conformal changes in the fusion protein that result in the insertion of a viral fusion peptide into the target cell membrane. This insertion is followed by further conformational changes within the fusion protein that bring the viral envelope and cell membranes into close proximity and results in the fusion of the two membrane bilayers.
A virus is unable to spread and propagate within its host if this fusion process is disrupted. Intentional disruption of this fusion process can be achieved by directing peptides and peptide mimics homologous to fusion protein sequences, antibodies that recognize the fusion protein, and other factors that act against the fusion protein.
Hemagglutinin 2 (HA2) an envelope protein of the influenza virus, an orthomyxovirus, is the prototypic RNA virus Class I fusion protein. HA2 contains an amino terminal hydrophobic domain, referred to as the fusion peptide, that is exposed during cleavage of the hemagglutinin precursor protein. Retroviral transmembrane proteins contain several structural features in common with the known structure of HA2 in addition to the fusion peptide, including an extended amino-terminal helix (N-helix, usually a “heptad repeat” or “leucine zipper”), a carboxy-terminal helix (C-helix), and an aromatic motif proximal to the transmembrane domain. The presence of at least four out of these five domains define a viral envelope protein as a Class I fusion protein.
FIG. 1 shows the five previously-described domains of the fusion proteins of the six families of Class I viruses. The fusion proteins originate in a hydrophobic fusion peptide, terminate in an anchor peptide, and incorporate an extended amino terminal alpha-helix (N-helix, usually a “heptade repeat” or “leucine zipper”), a carboxy-terminal alpha-helix (C-helix), and sometimes an aromatic motif proximal to the virion envelope. Also shown for each of the viral families is a sixth domain, referred to herein as the fusion initiation region (FIR), which was discovered by the present inventors and disclosed in U.S. Ser. No. 10/578,013.
About 10 to 20 percent of the population of the United States suffers from seasonal influenza each year. While most individuals recover from influenza in one to two weeks, the very young, the elderly, and persons with chronic medical conditions can develop post-flu pneumonia and other lethal complications. The causative agent of influenza is the influenza virus, an orthomyxovirus which readily develops new strains through a process of reassortment and mutation of the segmented viral genome.
Highly virulent strains of type A influenza virus can produce epidemics and pandemics. In recent years, there has been an emergence of a highly pathogenic strain of avian influenza A virus subtype H5N1 capable of inflicting a high mortality rate. Because of the threat posed by the influenza virus both to public health and as a potential agent of bioterrorism, developing therapeutics to control seasonal influenza and the increasing threat of pandemic influenza is a high priority.